Coded orthogonal frequency division multiplexing (COFDM) modulation is used in broadcasting (or multicasting or unicasting) broadband digital signals from a transmitter to a plurality of receivers. As one example, DVB-T (Digital Video Broadcasting-Terrestrial) system as defined in the standard ETSI (the European Telecommunications Standards Institute) EN 300 744 (Version 1.4.1) is a system in which coded orthogonal frequency division multiplexing (COFDM) modulation method is used in broadcasting (or multicasting) broadband digital television signals from a DVB-T transmitter to a plurality of DVB-T receivers.
Generally, in digital data communication orthogonal frequency division multiplexing (OFDM) is used, the COFDM only being one example thereof.
As the standard explains, before digital data (e.g. MPEG-2 (Moving Picture Experts Group) coded video, audio and/or data stream) is ready for transmission a plurality of operations are applied to it by the DVB-T transmitter. These operations comprise, among other things, outer coding by Reed-Solomon encoder, outer interleaving (i.e. convolutional interleaving), inner coding (with the aid of punctured convolutional code) and inner interleaving.
The inner interleaving is performed in an inner interleaver which comprises a set of bit interleavers for bit-wise interleaving followed by a symbol interleaver for symbol interleaving. The purpose of inner interleaving is to improve the system's tolerance to errors and interference by reordering the digital data to be transmitted in such a way that, basically, information contained in successive/adjacent digital data sequences will not end up to be transmitted by adjacent carriers. In that way interference occurring only occasionally in the frequency of only one or only a few data carriers can be coped with efficient error correction that the system provides.
In DVB-T system the transmitted signal is organized in frames. Each frame has a duration and consists of 68 OFDM-symbols. Each OFDM-symbol, in turn, is constituted by a set of carriers. The standard describes the usage of two modes of operation: a “2K mode” and an “8K mode. In 2K mode the number of carriers is 1705 and in 8K mode the number of carriers is 6817. However, only 1512 carriers of the total 1705 carriers in 2K mode and 6048 carriers of the total 6817 carriers in the 8K mode are defined as “active” carriers which actually carry the digital data (i.e. payload, useful data). The rest of the carriers are mostly for control purposes. It should be noted that by using another method of counting which takes a particular guard band into account the total number of carriers of 2048 (2K mode) or 8192 (8K mode) may be obtained. These numbers correspond to the used FFT (IFFT) size in each case (FFT=Fast Fourier Transform, IFFT=Inverse FFT). However, the number of active carriers is still 1512 (2K mode) or 6048 (8K mode).
Mapping of digital data (data words, also referred to as bit words or data units) onto the active carriers is performed in the inner interleaver. More particularly, this task is done by the symbol interleaver. A separate “2K symbol interleaver” has been defined for 2K mode and an “8K symbol interleaver” for 8K mode. In 2K mode, the 2K symbol interleaver maps 1512 data words (that is 12 groups of 126 data words, wherein the length v of each data word is v=2, 4 or 6 bits depending on the used modulation method)) coming from the set of bit-interleavers onto the 1512 active carriers of one 2K mode OFDM-symbol. Similarly, in 8K mode, the 8K symbol interleaver maps 6048 data words (48 groups of 126 data words) onto the 6048 active carriers of one 8K mode OFDM-symbol.
Recently, DVB-T system has been evaluated for situations for which it was not originally designed, such as mobile reception. Also, new usage scenarios of the system, such as IPDC (IP—Data Casting (Internet Protocol)) have different new requirements for the system. The use of the existing DVB-T 8K mode would probably offer wide enough coverage for mobile reception. However, the achieved mobility with the 8K mode might be too slow, i.e. reception might not succeed e.g. in a vehicle driving fast like at the speed of 120 km/h. The 2K mode, on the other hand, would offer sufficient mobility. However, a high density of base stations would be required with the 2K mode because the so called guard interval length is not long enough to support long transmitter distances in that mode. One proposed compromising solution to this problem is to define a new mode: a “4K mode”.
As to the inner interleaving of the proposed 4K mode, especially to the symbol interleaver, an obvious solution would be to define a new “4K symbol interleaver” which would map 3024 data words (24 groups of 126 data words) onto the 3024 active carriers of one 4K mode OFDM-symbol (it is probable that the number of active carriers would be 3024 in the 4K mode). However, this solution, of course, requires more space in the DVB-T transmitter and, particularly, in the plurality of DVB-T receivers since the new 4K symbol interleaver (or de-interleaver) has to be implemented in each device.
To overcome the above drawbacks there is need for a new method and system for communicating digital data and also need for new types of transmitters and receivers for implementation of the new method and system.